Project Summary/ Abstract The transcription apparatus undergoes liquid-liquid phase separation to form a biomolecular condensate at super-enhancer (SE)-regulated genes. Biomolecular condensates are currently being characterized throughout the cell, exchanging the traditional stoichiometric models of several biochemical mechanisms for super-stoichiometric models in which components of the condensate are concentrated and functionally compartmentalized into membrane-less organelles. A transcriptional condensate model has several implications for the basic molecular mechanisms of gene activation, as well as for our understanding of transcriptional dysregulation in cancer and in vivo target engagement in cancer therapy. In this research, we examine these implications through a study of the emergent physicochemical properties, driving interactions, and small molecule partitioning of Estrogen Receptor-mediated transcriptional condensates in breast cancer. Our preliminary data began to characterize these condensates in breast cancer tissue, cell lines, and in vitro. To build on these findings, we propose to use live-cell imaging and established condensate characterization assays to pursue the following aims: 1) Dissect the physicochemical properties of the Estrogen Receptor and Mediator in transcriptional condensates 2) Identify features important for the phase separation capacity of the Estrogen Receptor with Mediator 3) Build and utilize tools to characterize the properties of tamoxifen that determine its differential partitioning The work in this study will provide tools and a framework with which to consider future work on the impact of alterations to ER and/or MED1 on SE-mediated gene transcription in breast cancer cells. More broadly, by performing a deep dive into a well-studied TF-coactivator pairing in cancer, this work may inform analyses on lesser-studied cancers in which transcription is dysregulated, and a condensate model may provide insight or a therapeutic window. The proposed research will be conducted under the mentorship of Dr. Richard A. Young at the Whitehead Institute for Biomedical Research. The fellowship training plan will provide the trainee with skills in independent research, experimental techniques, scientific communication, leadership, and mentorship through one-on-one meetings with the sponsor, lab meetings, and presentation opportunities in and beyond the Whitehead and MIT Biology community.